Why can't fan art be the main image?

I'm not trying to be argumentative. I didn't know that rule, but I was wondering why can't fan art be the main image? Isn't all comic art, fan art? What counts as "official" art? Does it have to be made by the "official" artist at the time? Does the artist have to be famous? Furthermore if Greg Rucka draws a picture of Renee Montoya and posts it on a blog, is that a fan pircture? Isn't all art special and beautiful without some formal validity. Where does the area from "official" art to fan art begin and end? Isn't any picture not in a comic, technically fan art. Because if it was official it be bound by the comic. Or is it official promotional art? Even if Jack Kirby drew a picture of Superman, that'd technically it be fan art. Again I don't want to be viewed as me attacking the rule or anything. I just don't understand the validity of the rule.

@CEO_OF_FRESH: It's not encyclopedic. New users couldn't get a feel for the character through fan art. And many users would decide that their fan-art should be the main profile. Do you really want to see this as the main image for a page?

Just to add on what Ica said, it is also why fan fic is not in the wiki either. The wiki is meant to collect published comic information. They allow fan fic pics in galleries though I think that is a grey area.

The difference is fans vs professionals working for the publishers. If someone who is in the industry and has at any time worked for the publisher or worked on that specific character their art is not fan art it is official art that can be used. If it is a professional and they have never officially worked on the character or even worked for that publisher then they would be considered a fan in that instance and their art would not be official and can't be used. Fans are not comic professionals, their work is not published in the classic sense so they have no legal right to be making official art so it is fan art and not allowed to be used.

To be more clear the art we use is representing the characters and we must represent those characters in a way that the publisher/owner/creator would approve of and would use themselves if they wished otherwise it would be disrespectful to them and the medium we love.